Coating of Poly(arylene Sulfide), Fluoropolymer and Aluminum Flake

This invention relates to non-stick coating compositions particularly useful on cookware.

Poly(phenylene sulfide) resins are known for use in cookware coatings. Sometimes fluoropolymers and pigments, extenders or hardening agents such as titanium dioxide and aluminum dioxide are used. Although such coatings are useful on bakeware intended for use in an oven at lower and more uniform temperatures such as used in baking bread, they are generally not as hard nor as durable as desired for top-of-range cookware and for electrical appliances such as waffle irons.

The present invention provides a coating composition comprising, in weight percentage ranges based on the solids content, about:

(a) 95-35% poly(arylene sulfide) resin,

(b) 5-55% fluoropolymer polymerized or copolymerized from monomers selected from one or more monoethylenically unsaturated hydrocarbon monomers and hydrocarbon ether monomers, said monomers being completely substituted with fluorine atoms or a combination of fluorine atoms and chlorine atoms,

(c) 0.5-20% aluminum flake, and

(d) 0-40% titanium dioxide pigment, dispersed in liquid carrier.

The liquid carrier can be aqueous or organic. The poly(arylene sulfide) resin is preferably poly(phenylene sulfide) resin. The fluoropolymer is preferably polytetrafluoroethylene, but it can also be a perfluorinated copolymer of hexafluoropropylene and tetrafluoroethylene or a copolymer of tetrafluoroethylene and hydrocarbon ether monomers which have been completely substituted with fluorine atoms.

The invention also comprises articles of cookware, such as bakeware, coated with such coating compositions and heated to remove the liquid carrier and more or less coalesce the coating.

Applicant has discovered that aluminum flake can substantially increase the hardness at elevated temperatures of coating compositions of poly(arylene sulfide) resins and fluoropolymers, with optional titanium dioxide pigment. Such coating compositions have liquid carriers which are either aqueous or organic.

Applicable arylene sulfide polymers include polymers of the type which are prepared as described in U.S. Pat. No. 3,354,129. As disclosed in this patent, these polymers can be prepared by reacting a polyhalo-substituted cyclic compound containing unsaturation between adjacent ring atoms and an alkali metal sulfide in a polar organic compound. The resulting polymer contains the cyclic structure of the polyhalo-substituted compound coupled in repeating units through a sulfur atom. The polymers which are preferred for use in the coating procedure of this invention, because of their high thermal stability and availability of the materials from which they are prepared, are those polymers having the repeating unit --R--S-- where R is phenylene, biphenylene, naphthylene, biphenyl ether, or a lower alkyl-substituted derivative thereof. By "lower alkyl" is meant alkyl groups having 1 to 6 carbon atoms such as methyl, ethyl, propyl, isobutyl, hexyl, and the like. Preferably the arylene sulfide polymers have melting temperatures within the range of about 205.degree.-483.degree. C. If desired, the arylene sulfide polymer can be lightly cured, i.e., the molecular weight of the polymer can be increased by cross-linking or chain extension, e.g., by heating moderately, prior to application to the substrate.

A variety of fluoropolymers can be used, including those of U.S. Pat. No. 4,123,401 and U.S. Patent application Ser. No. 956,349, filed Oct. 31, 1978, equivalent to Patent Cooperation Treaty International Application US79/00807, filed Oct. 1, 1979, designating Japan along with other countries. All of the patents and applications cited herein are hereby incorporated by reference.

Preferably poly(phenylene sulfide) resin (PPS) is used in a weight ratio to fluoropolymer of about 80:20. Titanium dioxide pigment in an amount of about 15 parts by weight per 100 parts of poly(phenylene sulfide) resin plus fluoropolymer is helpful in further hardening the coating. The fluoropolymer is preferably either polytetrafluoroethylene (PTFE) such as an irradiated micropowder, or a copolymer of about 80 weight percent tetrafluoroethylene and 20 weight percent hexafluoropropylene (FEP).

Although various types of aluminum flake can be used in organic solvent, water tends to react with untreated aluminum flake, causing gassing. Thus, for aqueous compositions, preferably the aluminum flake has been reacted with phosphate ester or emulsified with fatty acid and neutralized to form a morpholine salt.

The invention will now be illustrated by examples. Parts, percentages and proportions herein are by weight except where indicated otherwise.

Prepare a premix by grinding in a pebble mill for 48 hours or grinding in a Dyno-mill for 5 passes a blend of:

This premix is then mixed in the following proportions:

The coating composition is applied to a substrate of any suitable material such as aluminum by suitable techniques such as spraying or roller coating. The coating is cured by heating 10 minutes at 400.degree. C. This drives off the liquid carrier and coalesces into a desirable coating. Alternately, the coating can be cured at 370.degree. C. for 15 minutes, at 425.degree. C. for 5-7 minutes or under equivalent conditions.

Make a premix as in Example 1 from:

This premix is then mixed in the following proportions:

These coating compositions are applied and cured as in Example 1.

Two coating compositions were prepared and applied as in Example 1, differing in that one contained the aluminum flake of Example 1 while the other did not. The hardness of the two coatings was compared at elevated temperatures by the Erichsen technique described, inter alia, in the article by Weinmann entitled "Ein neues Gerat zur Hartemessung von Antrichen und anderen Schutzuberzugen", published in Farbe und Lack, 68, No. 5, 323-326 (1962). The results in the following table demonstrate advantages of the invention.